1. Field of the Invention
The present invention relates to a wavelength tunable laser device suitable for multi-wavelength communication systems and so forth.
2. Description of the Related Art
Along with dramatic increase in demands for communication in recent years, development of wavelength division multiplexing systems (WDM systems), which realize high-capacity transmission by a single optical fiber by way of multiplexing plural signal beams of different wavelengths, shows progress. For such wavelength division multiplexing systems, a wavelength tunable laser capable of selecting a desired wavelength from a wide range of wavelengths is strongly expected in building the systems.
As the wavelength tunable laser with a wide band, one having a semiconductor optical amplifier (SOA) and a wavelength tunable filter arranged in a resonator is described in Patent Document 1 (Japanese Patent Application Laid-Open No. 2003-283024). This laser realizes oscillation of a single longitudinal mode by selecting one of longitudinal modes of the laser resonator arranged in small wavelength intervals through use of two filters, that are, a wavelength tunable filter and an optical filter having a periodical transmission wavelength (hereinafter referred also to as an etalon). For example, in an SOA-AOTF laser using an acousto-optic tunable filter (AOTF) as the wavelength tunable filter, the oscillation of a single longitudinal mode is obtained in a wide range of wavelengths (Δλ=90 nm) (Non-Patent Document 1 (K. Takabayashi et al. Proc. of ECOC 2003 vol. 4 890)). Control of oscillation wavelength of the laser described above is conducted by the following two controls in combination.
A first control is a control to adjust the wavelength selected by the wavelength tunable filter to a desired transmission peak wavelength of an internal etalon. A second control is a control to adjust the oscillation wavelength to a desired wavelength (for example, an ITU grid) by adjusting a longitudinal-mode position of the resonator.
Accordingly, it is necessary to configure the resonator such that its resonator length (phase) is controllable in this wavelength tunable laser so as to control the longitudinal-mode position of the resonator. An example of methods of controlling the phase is one having the steps of inserting a semiconductor layer therein and injecting current into the semiconductor layer to control its refractive index. Another example is one having the step of changing a position of a mirror constituting the resonator.
As the wavelength tunable laser device capable of accurately controlling the oscillation wavelength of the laser through use of the above-described two controls, such a device is proposed that has an integrated wavelength locker including two beam splitters, two photo-detectors (PD), and an optical filter with a periodical transmission wavelength (hereinafter, referred to also as an external etalon) in the external part of the resonator.
This conventional wavelength tunable laser, however, can achieve the expected purpose but has difficulty dealing with a further improvement in accuracy.